As a sensor of the theft and the like of an automobile generating a signal by sensing a vibration at the time of the theft and the like of the automobile, there is already provided a sensor shown in FIG. 7 wherein the reference numeral 1 represents a housing body, 2 represents an elastic string provided as stretched to the housing body 1, 3 represents a weight provided in the middle of the elastic string 2 and provided with a permanent magnet 4 exposed on the lower surface and a copper winding coil 5 is provided below the above mentioned weight 3.
This sensor is fitted to a proper part of an automobile. The principle of the vibration of this sensor is of a torsion bar system with the elastic string 2 as an axis so that, when the automobile vibrates, the weight 3 will become a pendulum swinging with the elastic string 2 as an axis, the magnetic flux intersected with the copper winding coil 5 by the permanent magnet 4 of the weight will vary, an induction current will be generated in the copper winding coil 5 and the theft and the like of the automobile will be thereby sensed.
However, with this sensor, there has been a defect that, in such case that the automobile is greatly inclined as parked, when the surface on which the above mentioned copper winding coil 5 is arranged greatly inclines, the weight 3 will be rotated to be directed in the perpendicular direction by the gravity, the permanent magnet 4 will separate away from the copper winding coil 5 and therefore the sensitivity will reduce. Also, there has been a defect that, in case the car is parked above high voltage lines embedded under the ground or in case a transmitting antenna generating strong electromagnetic waves is located in the vicinity, an induction current will be generated in the above mentioned copper winding coil 5 under the influence of an electromagnetic induction from outside and the sensor will mis-operate or will be made inoperative by an excess input.